Numbers: The Nervous System, From 268-MPH Signals to Trillions of Synapses

268 Speed (in miles per hour) at which signals travel along an alpha motor neuron in the spinal cord, the fastest such transmission in the human body. Sensory receptors in the skin, which lack the speed-boosting insulating layer called a myelin sheath, are among the slowest, at 1 mph.

100,000 Miles of myelin-covered nerve fibers in the brain of an average 20-year-old. Neuroscientists at UCLA, who have studied myelination in the brains of adults ages 23 to 80, reported in September that the coating peaks around age 39—the same age at which participants hit top speeds in standard tests of motor abilities.

100 trillion Minimum number of neural connections, or synapses, in the human brain. That is at least 1,000 times the number of stars in our galaxy. British researchers reported in December that genes involved in the workings of synapses account for about 7 percent of our genome.

50 Depth, in nanometers, of the smallest grooves detectable by a human fingertip (that is about 2 millionths of an inch). Most of the 2 billion or so nerve endings in the outermost layer of our skin sense pain; those dedicated to temperature allow us to detect differences as small as 0.01 degree Fahrenheit.